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9.已知多项式$A=x^2+xy+3y$，$B=x^2-xy$，若$(x-2)^2+|y+5|=0$，求$2A-B$的值.
答案
9. $-56$
10.已知$A=(m-5)x^n y$，$B=-\dfrac{1}{3}x^2 y +6$，若$A+B=6$，求代数式$\dfrac{m}{n^4}$的值。
答案
10. $\dfrac{1}{3}$
【解析】$A+B=(m-5)x^ny+(-\dfrac{1}{3}x^2y +6)=(m-5)x^ny-\dfrac{1}{3}x^2y+6. \because A+B=6,\therefore (m-5)x^ny-\dfrac{1}{3}x^2y=0,\therefore m-5=\dfrac{1}{3},n=2,$
解得$m=\dfrac{16}{3},n=2. \therefore \dfrac{m}{n^4}=\dfrac{\dfrac{16}{3}}{2^4}=\dfrac{1}{3}.$
【解析】$A+B=(m-5)x^ny+(-\dfrac{1}{3}x^2y +6)=(m-5)x^ny-\dfrac{1}{3}x^2y+6. \because A+B=6,\therefore (m-5)x^ny-\dfrac{1}{3}x^2y=0,\therefore m-5=\dfrac{1}{3},n=2,$
解得$m=\dfrac{16}{3},n=2. \therefore \dfrac{m}{n^4}=\dfrac{\dfrac{16}{3}}{2^4}=\dfrac{1}{3}.$
11. 当$2x^2 + xy = 6$，$3y^2 + 2xy = 9$时，求代数式$4x^2 + 8xy + 9y^2$的值.
答案
11. $39$
【解析】$4x^2+8xy+9y^2=2(2x^2+xy)+3(3y^2+2xy)=2×6+3×9=39.$
【解析】$4x^2+8xy+9y^2=2(2x^2+xy)+3(3y^2+2xy)=2×6+3×9=39.$
